Technology for manufacturing large-area photovoltaic cells from amorphous semiconductor alloys has been developed in recent years. Breakthroughs have been made in preparing amorphous semiconductor materials of electronic device quality. These high quality materials include hydrogen, fluorine, or a combination of the two in amorphous silicon, silicon-germanium, and germanium. It is believed that hydrogen and fluorine passivate or satisfy dangling bonds and other structural defects in the amorphous structure so that desirable charge carrier transport properties are achieved.
The principal method of preparing these amorphous semiconductor materials is glow discharge deposition. In that process, a gas mixture containing at least one semiconductor precursor gas, such as silane, disilane, silicon tetrafluoride, germane, and germanium tetrafluoride or mixtures of them, is admitted to a vacuum chamber held at a reduced pressure, typically 13 to 65 pascal. The gas mixture may also include hydrogen or argon as a diluent and a dopant precursor gas, such as diborane or boron trifluoride to deposit a p-type conductivity material or phosphine or phosphorous tetrafluoride to deposit an n-type conductivity material. The gas mixture also includes a source of hydrogen and/or fluorine. Material deposited without the presence of a dopant precursor gas is typically slightly n-type in conductivity, is substantially intrinsic and may be compensated to a higher resistivity with a trace of a p-type dopant, such as boron.
The gas mixture is admitted to the chamber through a fixture that forms a cathode. A glow discharge plasma is struck between the cathode and an electrically conductive substrate by impressing an electrical potential across the cathode and substrate. The glow discharge plasma is sustained by electrical power that may be direct current or may be alternating current up through the microwave frequency range. The glow discharge disassociates the gas mixture into various species that deposit on the substrate and build up the depositing alloy. By changing dopant precursor gases during the deposition process, p-n, p-i-n, and more complex device structures may be deposited. Three layer p-i-n and multiple p-i-n amorphous silicon alloy and amorphous silicon-germanium alloy structures have proven particularly useful as photovoltaic devices.
The process of producing glow discharge deposited amorphous photovoltaic devices has been developed to permit continuous production of such materials over large areas. For example, methods for the continuous production of amorphous photovoltaic material on large-area, flexible metallic substrates has been disclosed in U.S. Pats. Nos. 4,400,409 to Izu et al. for Method of Making P-Doped Silicon Films; 4,410,558 and 4,519,339 to Izu et al. for Continuous Amorphous Solar Cell Production System; 4,485,125 to Izu et al. for Method of Continuously Producing Tandem Amorphous Photovoltaic Cells; 4,492,181 to H. Ovshinsky et al. for Method for Continuously Producing Tandem Amorphous Photovoltaic Cells; and 4,514,437 to Nath for Apparatus for Plasma Assisted Evaporation of Thin Films and Corresponding Methods of Deposition. The disclosures of these patents are incorporated herein by reference. Apparatus for depositing complex amorphous semiconductor alloy devices on flexible substrates 30 cm. wide and over 300 m. long has been built and is now operating.
More recently, very lightweight amorphous semiconductor alloy arrays of photovoltaic cells have been constructed from continuously deposited alloy materials. These lightweight cells have an exceptionally high specific power, i.e. power output to mass ratio. The lightweight cells are prepared in the way described above, but on a very thin substrate, such as electroformed metal foil, or a metal substrate that is chemically etched to an unconventional thinness, or on an insulator initially supported by a metal substrate that is completely removed by chemical etching after deposition of the amorphous alloy. See U.S. patent application Ser. No. 696,390 filed Jan. 30, 1985, by Hanak for Extremely Lightweight, Flexible Semiconductor Device Arrays and Method of Making Same. It is desirable to fabricate these extremely lightweight arrays directly from continuous processing machinery rather than to thin or remove a conventional thickness substrate in order to reduce the number of process steps and thereby to improve yield and to reduce cost. It is also desirable to avoid use of a very thin electroformed foil because of the special care required in handling that delicate foil.